The decomposition of organic matter in landfills produces significant amounts of gas, primarily methane and carbon dioxide, along with trace amounts of other organic gases and certain contaminants. When landfill gas migrates through soil or is released into the atmosphere it presents safety hazards related to the potential to form explosive mixtures of methane and air, and environmental hazards related to the release of methane and other pollutants. Landfill gas can also create nuisance odors within and beyond the landfill boundaries. For these reasons, federal and state regulations require that landfill owners provide positive means to control migration and release of landfill gas. Accordingly, gas collection wells are usually placed vertically in a landfill to collect the gases produced during the decomposition process, and these wells are connected together by a gas pipeline system that transports the collected gas including the entrained contaminants to a convenient location for beneficial use or disposal.
Disposal of the landfill gas is normally accomplished by burning the gas within an enclosed or open flare. Beneficial use of landfill gas can take a variety of forms with the most common being fuel for engines that generate electricity, fuel for landfill leachate evaporation systems, or direct sale of the gas for off-site applications such as fuel for industrial boilers or electrical generators. Government regulations dictate at what temperatures the gas must be burned and for how long the gas must be exposed to the prescribed temperatures based on air quality standards. The regulations are designed to assure that the gas and the contaminants therein are destroyed prior to being released to the atmosphere. Where regulations require the use of an enclosed flare, the landfill gas is typically burned at the bottom of the flare stack, which is designed to maintain the gas undergoing treatment in the combustion process at a relatively high temperature (e.g., usually around 1500° F., typically between 1400° F. to 1800° F. and in some cases between 1200° F. and 2200° F.). The volume of the flare stack is selected to provide enough residence time, such as between 0.3 and 1.5 seconds, to ensure adequate treatment of the components within the gas. The difference in temperature from the bottom of the flare stack to the top of the flare stack is normally quite small, meaning that the exhaust gas ejected out of the top of the flare stack is still very hot and thus contains significant heat energy. Likewise, due to inherently poor thermodynamic efficiency, both internal combustion engines and turbines fueled by landfill gas eject significant heat energy to the atmosphere in the form of exhaust gas at temperatures that are typically in the range of 750° F. to 1150° F. and almost always in the range of 600° F. to 1200° F. Because this energy is simply released to the atmosphere, it is referred to as waste energy or waste heat. Where exhaust gas is at a relatively high temperature such as 600° F. to 2200° F. and the quantity of the hot gas is such that the total energy content amounts to all or a significant portion of that required to operate a desirable downstream process, opportunities exist to beneficially use the waste heat. Regardless of whether a gas is simply flared or employed within a process for beneficial use, very few systems are designed to recover and beneficially utilize any of the waste heat exiting a flare stack or combustion engine at, for example, a landfill.